1. Field of the Invention
This invention is directed to the field of polymers, in particular, to polymers used as protective coatings in electronics.
2. Background of the Invention
Microelectromechanical systems, or MEMS, is an emerging technology that may fundamentally affect every aspect of our lives. The hallmark of the next thirty years of the silicon revolution will likely be the incorporation of new types of functionality onto microelectronic devices including structures that will enable the devices to think, sense, act and communicate. This revolution will be enabled by MEMS. Currently, automobile accelerometers, medical equipment, and sensing systems utilizing MEMS technology have already been commercialized, and fiber communication technology will accommodate a large potential market for the use of MEMS devices in the immediate future. However, one of the major issues of today's MEMS technology is the packaging that provides mechanical and environmental protection to MEMS devices. Unlike integrated circuits (ICs), the packaging for MEMS is much more difficult, because most MEMS devices have complex topography and delicate moving parts which need to be protected but not affected/hampered by the package. Accordingly, there is a need for materials having unique properties enabling them to protect MEMS devices.
Examples of sensors that need such protection include a multi-chip module containing a piezopressure sensor fabricated with MEMS technology and analogue/digital driving IC's. The most challenging task, besides conventional packaging issues, is to protect the MEMS sensor against adverse environmental conditions without affecting the moving parts and sensitivity. All the sensors typically experience extreme temperature variation (−55˜140° C.), mechanical shock and vibration, high humidity, jet fume contamination, and UV radiation, etc. Therefore, a multifunctional conformal coating capable of withstanding these conditions is needed. In terms of the concerns from different aspects of aerospace and avionics applications, to monitor the static pressure on the wings, tail, nacelle, engine, and other interested sites of the body of an aircraft during flight test, the desired conformal coating should be of excellent resistance to moisture ingress, mobile ion (such as Na+, K+, Li+, Cl−) permeation, low internal stress, good adhesion, planarization as well as good resistance to jet fuel contamination.
Generally, many polymeric resins have been used commercially for microelectronic device encapsulation. Epoxy resins and silicone elastomers have been used most extensively to provide the requisite characteristics required by the electronics industry. Yet, in spite of the popularity that epoxy resins and silicone elastomers have enjoyed commercially as microelectronics device glob-top or potting resins, epoxy resins and silicones elastomer prepared for MEMS based multichip module of aerospace and avionics applications is still a new and challenging area.
Epoxy resins have excellent adhesion properties and resistance to aggressive chemicals. However, high cross-linking density induced stress is a well-known key factor in the failure of fragile microelectronic devices and the stress sensitive MEMS based devices. Furthermore, the extreme working temperature for aerospace application is from −55° C. to 140° C., and no obvious mechanical property change can be allowed in this temperature window. No qualified epoxy resin is currently available that can withstand these adverse conditions.
Silicones, as typically pliable elastomer materials, can function as durable dielectric insulation, as barriers against environmental contamination, and as stress relieving shock/vibration absorbers over a wide humidity/temperature range. However, the intrinsic low polar structure of silcones lowers their contamination resistance to contamination including aliphatic hydrocarbons, for example, octane, etc. Currently, commercially available silicone elastomers can not meet the aforementioned requirements either.
Accordingly, there is a need for a low stress epoxy, having an internal stress from thermal cycling and manufacturing which will not influence the sensing accuracy of a MEMS based pressure sensor.
There is another need for a low stress epoxy with a glass transition temperature (Tg) lower than −60° C.
There is still another need for a low stress epoxy with low viscosity and self air degassing.
There is yet another need for a silicone elastomer, preferably a two-component elastomer, with improved jet fume contamination resistance, a Tg of at least about −60° C. to about −120° C.
There is another need for a two-component silicone elastomer with room temperature curability.
There is another need for a low viscosity two component silicone elastomer with good wetting and self air degassing characteristics for avoiding any voids trapped during processing.